In the field of electrophotography, a toner image is first formed on a photosensitive member, transferred onto a transfer paper, and then, fixed thereon to form an image. There is a technique as disclosed in Japanese Patent Laid-Open (Kokai) H2-181166, which is hereby incorporated by reference, in which a toner image is formed on a belt-like toner image holder which is then transferred and fixed onto a sheet of paper. The belt-like image holder is held in a triangular configuration.
Such a recording technique, however, requires a dry powder toner, which complicates its handling; it also requires a relatively large device which is not suitable for a personal machine.
For these reasons, another technique has become popular in the electrophotographic field. This technique is ink jet printing. A conventional ink jet printer, which uses pigmented printing ink however, has had drawbacks of its own in poor image durability and exposure resistance.
An image-forming "jet flow" apparatus has been disclosed in an international publication No. W093/11866, which is hereby incorporated by reference, in which pigmented particles are used as the coloring material to overcome the above-mentioned drawbacks. This apparatus is equipped with a conductive supply tube; a predetermined voltage is applied onto the supply tube to form a predetermined electric field between the supply tube and the electrodes facing the end of the tube. A liquid dispersion of pigmented particles charged to the same polarity as the electrical potential of the supply tube is supplied to the tube. The pigmented particles separate from the liquid. There is a recording medium, such as paper, interposed between the end of the supply tube and the facing electrodes, which is transported in a predetermined direction.
Because of the electric field formed between the end of the supply tube and the facing electrodes, the colored particles in the liquid dispersion are electrostatically attracted by facing electrodes at the ejection point near the end of the supply tube; and a semi-spherical liquid meniscus is formed at or near the end of the supply tube. Since, however, the particles cannot flow from the liquid meniscus because of the surface tension of the liquid in the dispersion, they stay at the liquid meniscus. In this manner, many particles remain at the point of the liquid meniscus as agglomerates.
As the recording medium (paper) is transported in a predetermined direction, a higher voltage is applied to the supply tube, the electric field is established between the end of the supply tube and the facing electrode is strengthened. The electrostatic attraction force on the particulates or agglomerates becomes stronger than the surface tension of the liquid of the dispersion; and consequently, the agglomerates or particles pass through the liquid meniscus and flow toward the recording medium which is positioned between the supply tube and a facing electrode. A predetermined image is formed on the recording medium with agglomerates of colored particles from the supply tube, having been freed from the liquid meniscus.
Since the image-forming apparatus configured based on the above "jet flow" theory does not use a nozzle which establishes droplet size in a conventional ink jet printing, pigmented particles can be used. Therefore, problems, such as image durability and exposure resistance, of conventional ink jet printers can be resolved.
Furthermore, in a conventional jet flow image-forming apparatus, a recording medium (such as paper) is transported between the end of the supply tube and the facing electrodes. It is therefore necessary to provide a predetermined gap therebetween to allow the recording medium to pass through smoothly. An insufficient gap may cause poor image formation due to contact with the recording medium.
Consequently, since such a predetermined gap needs to be provided between the end of the supply tube and the facing electrodes in a conventional image-forming apparatus, a relatively high voltage must be applied to the supply tube in order to form an electric field strong enough to transport the particle agglomerates. The high voltage which the supply tube requires makes the integration of the drive circuit for the supply tube difficult, the configuration remains complex, and consequently increases the difficulty of manufacture and cost.
If the gap size is set according to the thickness of the recording medium to be used, a relatively thin recording medium must be used for narrowing the gap as much as possible to reduce the recording voltage required for printing; consequently, this limits the freedom of selection of the recording medium.
On the other hand, if the gap size is made constant and printing paper of different thickness and different electric resistance is used, even if a predetermined voltage is applied to the supply tube, the magnitude of the electric field formed between the end of the tube and the facing electrodes can vary, preventing stable image formation.